1. Field of the Invention
The present invention relates to a mist ejection head and an image forming apparatus, and more particularly, to a mist ejection head and an image forming apparatus for liquid having high viscosity.
2. Description of the Related Art
Japanese Patent Application Publication Nos. 62-85948, 62-111757, 2002-59549, and 2002-166541 each disclose a liquid ejection apparatus which ejects fine liquid droplets in the form of a mist.
The ejection of mist is performed by creating a mist of the liquid by means of an ultrasonic wave. More specifically, in general, atomization caused by cavitation (hollowing), or atomization caused by capillary surface waves, are used. If the latter type of method is used, then it is possible to generate a mist of uniform particle size and the energy efficiency is good.
In a case where a planar wave is applied toward the free liquid surface according to the method of the capillary wave atomization, provided that the frequency of the ultrasonic wave (planar wave) and the amplitude (onset amplitude) of the ultrasonic wave on the free surface of the liquid (the liquid-atmosphere interface, which is also commonly called “meniscus”) satisfy particular conditions relating to the properties of the liquid, then the surface tension wave at the meniscus of the liquid oscillates in a time series. Consequently, very small liquid droplets (mist) break away from the wave peaks of the surface tension wave at the free surface of the liquid, at certain time points.
However, in a mist ejection head in the related art which ejects mist of this kind, as indicated by the capillarity number which is the ratio between the viscosity of the liquid and the capillary effect, it is, in principle, difficult to eject liquid of high viscosity. Even when an ultrasonic wave generating element operates at 10 MHz or above, the viscosity limit for generating a mist is 2 to 3 cP.